Flatworms
Phylum Platyhelminthes
 There are hundreds of species of flatworms
that have been identified to date.
 They are assigned to phylum
Platyhelminthes.
 Prefix platy means “flat”
 Suffix helminth means “wormlike”
Phylum Platyhelminthes
 Flatworms have a solid body construction
 No body cavity between their gut and skin
 Animals with no body cavity between their gut and body
wall are termed acoelomate.

Acoelomate animals have a limited amount of organ
development and are considered simple animals.
Phylum Platyhelminthes
 When they evolved, flatworms were the first animals
to have a middle embryonic cell layer, or mesoderm.
 Thus, they (and higher phyla) have three layers in their
embryo.
Phylum Platyhelminthes
 As you recall, mesoderm
makes possible the
formation of muscle and
other connective tissues,
blood vessels, and organs
such as the testes and
ovaries.
Phylum Platyhelminthes
 As a result of their mesoderm, flatworms possess a fair
amount of muscle tissue and a reproductive system
comparable to that of higher animals.
 They do not however, have any blood vessels or
circulatory system.
 Materials are circulated through their body by
diffusion.
 Other systems flatworms do not include a respiratory
system. Oxygen and carbon dioxide are exchanged
across their skin by diffusion.
Phylum Platyhelminthes
 Flatworms were the first animals to be bilaterally
symmetrical.
 They were also the first animals to develop a distinct
head region containing sensory organs.
 Animals that have ahead containing a concentration of
nerve cells are said to be cephalized.
 Cephalization enabled them to adapt to their
surroundings more successfully.
 Note: Zoologists credit them with the first eyespots
Phylum Platyhelminthes
 Having no coelom nor circulatory system, flatworms
must be thin enough so that dissolved substances,
such as oxygen and carbon dioxide, can pass through
their solid body by diffusion.
 A thin body shortens the distance these substances
must travel to reach each cell.
 To help shorten the distance food substances must
travel, the gut in a flatworm is highly branched.
Phylum Platyhelminthes
 Portions of it run close to
practically all of the flatworm’s
tissues, giving each cell access to
food molecules.
 The gut has only one opening –
the mouth – through which food
enters and undigested food exits.
 Thus, like the animals we studied
before flatworms, the digestive
system is primitive.
Phylum Platyhelminthes
 There are three main classes in phylum
Platyhelminthes:
 The latter two classes consist of flatworms that live as
parasites.
Turbellaria
2. Trematoda
3. Cestoda
1.
Phylum Platyhelminthes
 Trematoda and Cestoda are parasites.
 This means that they live in and obtain their nutrition
from a host organism
 They cause untold misery and bring early death to
immense numbers of humans, especially in the poorer
parts of the world.
Class Turbellaria (Planarians)
 Flatworms of the class Turbellaria are described as
independent free-living.
 This means that they move about and feed on their own.
 Depending on the species, turbellarians can be found
living in aquatic environments, damp soil, or fresh
water.
Dugesia tigrina
Tricladida
Class Turbellaria (Planarians)
 The most commonly studied member of class




Turbellaria is the small freshwater flatworm
planarians.
Several species of this flatworm have been identified.
They can be found under rocks and logs in streams
and ponds.
They live partly as scavengers
and partly as predators.
They grow to 20mm or less.
Class Trematoda (Flukes)
 Flatworms of the class Trematoda are known as flukes.
 They are all parasitic and may be very damaging to their
hosts.
 The Asian liver fluke is very common in many parts of Asia
and is responsible for a great deal of sickness, disability and
death.
 People become infected by
eating raw or improperly
cooked meat carrying these
fluke larvae.
Class Trematoda (Flukes)
 Other species of flukes invade the lungs, spleen, intestines,




or blood.
Schistosomes, or blood flukes, infects at least three
hundred million humans in tropical regions.
These tiny worms drill through a person’s skin, enter the
blood stream, and lay hundred of eggs.
The eggs can damage or block small blood vessels and
often cause internal bleeding in the bladder and intestine.
Schistosomiasis kills approximately 600 thousand people
in Asia, Africa, Latin America, and the Middle East.
Class Cestoda (Tapeworms)
 Flatworms of the class Cestoda are known as
tapeworms because of their shape.
 These parasitic flatworms infect nearly almost all
vertebrates.
 The adult stage lives in the intestine of its host.
 It depends upon the host not only for food but for the
digestion process as well, as it lacks a digestive system.
Class Cestoda (Tapeworms)
 The head of a tapeworm is known as a scolex and is
equipped with one or more kinds of attachment
organs, depending upon the species.
 The segments making up the rest of the body are
called proglottids.
Class Cestoda (Tapeworms)
 These structures contain both male and female
reproductive organs and produce packets of fertilized
eggs.
 As a tapeworm grows, it adds proglottids just behind
the neck, pushing the older proglottids toward the
end. The proglottids break off and pass out of the
intestine with the feces.
Class Cestoda (Tapeworms)
 Tapeworms that can infect humans include the beef
tapeworm and pork tapeworm.
 People become infected when they eat undercooked
meat containing the larvae of these worms.
Venn Diagram
Flatworms
Cnidarians
Cnidarians
Sponges
Radial symmetry
Mesoglea
No organs or organ
systems
Polyp/medusa
Tentacles/stinging cells
Endoderm
Ectoderm
Hermaphrodite
Regeneration
Gastrovascular
cavity
Bilateral
Mesoderm: Acoelomate
Organ
Organ system: reproductive
Ganglia: eyespot